Drafting boards are frequently provided with a working surface formed by a sheet of vinyl (polyvinyl chloride), that material being selected because of its resilience, toughness, and durability. To insure that the working surface is precisely planar and remains so, such a vinyl sheet is permanently bonded to the planar top surface of the board or panel. Such a construction avoids the possibility of bulges or unevenness that would make the working surface unacceptable for drafting purposes; however, it also renders the covering sheet non-removable should replacement be desired because of damage, discoloration, or wear.
A typical drafting board also includes metal or plastic trim pieces that are applied to the periphery to form a rim or border. Ordinarily, the top surface of the trim is disposed at or below the working surface of the board so that the trim does not interfere with use of that surface in drafting operations. Various means are disclosed in the art for securing trim pieces in place, not only for drafting boards but also for counters, desk tops, and the like. Reference may be had to U.S. Pat. Nos. 2,853,750, 2,228,504, 1,496,581, 2,251,395, 2,957,737, 2,793,090, and 3,389,520 as illustrative of prior art constructions.
Initial efforts to provide a drafting board with a strippable and renewable covering of vinyl or other similar polymeric sheet material, and also with a peripheral rim below the plane of the working surface, have been unsuccessful because conventional pressure-sensitive adhesives are insufficient to prevent lifting or bulging of the covering along the outer limits of the board's top surface in response to forces of extended duration imposed on the cover sheet by the peripheral rim and because of the characteristic stiffness of a material suitable for use as a drafting board cover sheet. While such bulges could be avoided by utilizing an adhesive other than a pressure-sensitive adhesive, such a substitution would also prevent the covering from being easily stripped away when replacement is desired. Accordingly, this invention is concerned with a drafting board construction which utilizes a replaceable cover sheet that maintains a planar working surface, without bulges or other deformations, and yet may be easily stripped away when removal and replacement are desired.
An important aspect of this invention lies in the discovery that if the cover sheet of vinyl, or similar polymeric material having a durometer within the range of about 50 to 90 on the Shore C scale, slopes gradually downwardly from the outer limits of the panel and into a peripheral recess or depression formed in that panel, the edge of the covering sheet may be secured or protected by a suitable retaining rim without weakening and disrupting the adhesive connection between the cover sheet and panel to produce bulges or irregularities in the working surface. Specifically, the cover sheet should slope from the planar top surface of the panel to the inner edge of the retaining rim at an angle no greater than about 45 degrees, preferably within the range of about 15 to 40 degrees. Such a construction allows the use of a cover sheet of sufficient thickness (about 0.015 to 0.045 inches) to provide the resilience and durability needed for drafting board usage.
Briefly, the board takes the form of a rigid rectangular panel having a planar top surface and having a peripheral depression defining an upwardly-facing border surface which extends along a plane spaced below the planar top surface. The resilient polymeric cover sheet, preferably of polyvinyl chloride, of the durometer and thickness indicated above, extends over the top surface and over the border surface and is strippably secured to the top surface by a suitable pressure-sensitive adhesive. A peripheral retaining rim extends about the panel, the rim having planar upper portions extending over the cover sheet within the depression, intermediate side portions extending about the side surfaces of the panel, and bottom portions extending about the outer limits of the panel's underside. Fasteners extend through the bottom portions of the rim and engage the panel for securing the rim in place and for retaining the periphery of the cover sheet within the peripheral depression of the panel. The planar upper portion of the rim and the cover sheet together have a combined thickness substantially less than the distance between the planes of the top surface and border surface of the panel. Also, the planar upper portion of the rim has a width that is substantially less than the width of the depression so that its inner edge is spaced from the outer limits of the panel' s planar top surface a distance greater than the distance between the planes of the top and border surfaces. In a preferred embodiment, the fasteners used to hold the retaining rim in place take the form of screws received in threaded openings in the bottom portion of the rim. As the screws are tightened against the undersurface of the panel, the rim is lowered to clamp the periphery of the cover sheet tightly within the panel's peripheral depression.
Other features, objects, and advantages of the invention will become apparent from the specification and drawings.